Demand for high resolution microscopy and spectroscopy in the visible optical regime has grown at a brisk pace, fueled by both fundamental research and technological developments. With increasing attention focused on the physics and chemistry of microstructures and submicron devices, improved optical devices are required which provide spectroscopic and imagine capabilities at increasingly fine resolutions.
Traditional optical imaging devices are based on so-called "far-field" optics, where the distances between the illumination source, the sample, and the detector are much larger than the wavelength of light is, and a focusing element such as a lens is employed. The resolution of far-field optics is fundamentally limited to the diffraction limit of .lambda./(2 N.A.), where N.A. is the numerical aperture of the focusing lens. As a result, far-field optical imaging devices are ineffective in applications requiring a resolution finer than .lambda./2.
Recent developments in increasing the resolution of optical imaging devices have focused on so-called "near-field" optics or near-field microscopy. In near-field imaging, a subwavelength aperture is scanned at a height of a few tens of a nanometer above a sample to be observed, and the intensity of reflected or transmitted light is recorded at each point of the sample. The near-field imaging technique is described, for example, in an article by D. W. Pohl entitled "Scanning Near-Field Optical Microscopy," in Advances in Optical and Electron Microscopy, Vol. 12, pp. 243-312 (1991). The size of the aperture is the dominant factor in determining the resolution of a near-field optical imaging device. Near-field optics are employed in near-field scanning optical microscopy ("NSOM"), for example, and can be used to provide finely detailed imaging at resolutions which are finer than half of the wavelength of light used in the device (i.e. "subwavelength resolution").
One common near-field optical imaging device employs a single-mode optical fiber with a tapered tip. The tapered tip of the fiber forms an aperture of about 100 nm at the end, as described in an article by E. Betzig et al. entitled "Near-Field Optics: Microscopy, Spectroscopy, and Surface Modification Beyond the Diffraction Limit," in Science, Vol. 257, pp. 189-195 (Jul. 10, 1992). The sides of the tip are coated with a thin metallic film in order to confine the light inside the tip and allow it to exit only from the aperture. The need to metallize the taper arises because the dielectric mode (light) confined in the core of the fiber in the tapered region is not guided efficiently inside the core and has substantial leakage into the cladding. A similar technique is described in U.S. Pat. No. 5,633,972 to Walt et al., in which each of a plurality of imaging fibers is tapered and coated with a thin opaque metal at the tapered end.
Near-field optical imaging devices provide vastly improved resolution in comparison to far-field optical imaging devices. Indeed, among near-field and far-field devices, only near-field optical imaging devices are capable of providing subwavelength resolution. However, conventional near-field optical imaging devices suffer from several drawbacks.
First, due to the very fine resolution of near-field optical imaging devices caused by the narrow fiber tip, only a small area of a sample can be observed by conventional near-field optical imaging devices at any one time. In order to generate an image of a particular area of a sample, the fiber tip of a conventional near-field optical imaging device must therefore be systematically scanned over many discrete points of the desired area, a technique known as "raster scanning." As a result, conventional near-field optical imaging devices require a relatively complex positioning system which is capable of carrying out raster scanning by moving the fiber tip in extremely small increments and positioning the fiber tip within tight tolerances with respect to the sample. Thus, conventional near-field optical imaging devices requiring raster scanning are expensive and difficult to manufacture, especially when large areas (greater than several micrometers) are to be scanned. Moreover, conventional near-field optical imaging devices generate images of sample areas slowly due to the sequential imaging of small, discrete points of sample areas required by the raster scanning technique.
Second, the conventional near-field optical imaging devices described above require special processing to deposit the thin metallic layer at a tapered tip portion of each optical fiber. The application of this metallic coating renders the production of such conventional near-field optical imaging devices more complex and costly, especially given the tight tolerances required for imaging fiber tip apertures which are smaller than the wavelength of light. Moreover, the metallized near-field optical imaging devices of the prior art exhibit limited transmission efficiency because of substantial loss in the tapered region.
Accordingly, what is needed is a near-field optical imaging device that: (a) provides subwavelength resolution; (b) is relatively easy and inexpensive to manufacture in comparison to prior art metalized-tip devices; and (c) provides an enhanced field of view, thereby minimizing the need for raster scanning.